The present invention relates to a powder including agglomerated hexagonal boron nitride platelets, a method of making the powder, and its use in a polymer blend.
Microelectronic devices, such as integrated circuit chips, are becoming smaller and more powerful. The current trend is to produce integrated chips which are steadily increasing in density and perform many more functions in a given period of time over predecessor chips. This results in an increase in the electrical current used by these integrated circuit chips. As a result, these integrated circuit chips generate more ohmic heat than the predecessor chips. Accordingly, heat management has become a primary concern in the development of electronic devices.
Typically, heat generating sources or devices, such as integrated circuit chips, are mated with heat sinks to remove heat which is generated during their operation. However, thermal contact resistance between the source or device and the heat sink limits the effective heat removing capability of the heat sink. During assembly, it is common to apply a layer of thermally conductive grease, typically a silicone grease, or a layer of a thermally conductive organic wax to aid in creating a low thermal resistance path between the opposed mating surfaces of the heat source and the heat sink. Other thermally conductive materials are based upon the use of a binder, preferably a resin binder, such as, a silicone, a thermoplastic rubber, a urethane, an acrylic, or an epoxy, into which one or more thermally conductive fillers are distributed.
Typically, these fillers are one of two major types: thermally conductive, electrically insulative or thermally conductive, electrically conductive fillers. Aluminum oxide, magnesium oxide, zinc oxide, aluminum nitride, and boron nitride are the most often cited types of thermally conductive, electrically insulative fillers used in thermal products. Boron nitride is especially useful in that it has excellent heat transfer characteristics and is relatively inexpensive.
However, in order to achieve sufficient thermal conductivity with presently used fillers, such as boron nitride, it has been necessary to employ high loadings of filler in the binder (see, e.g., U.S. Pat. Nos. 5,898,009, 6,048,511, and European Patent No. EP 0 939 066 A1, all to Shaffer et al., which teach an alternate methodology to achieve solids hexagonal boron nitride loading approaching 45 vol. %. This method involves: (a) crushing high purity hexagonal boron nitride powder into particles extending over a size range of at least 100 microns, (b) cold pressing the crushed particles into a compacted form, (c) granulating the compacted form into a granulated powder, and (d) cold pressing the resulting powder, with the operations of cold pressing and granulation occurring in one or more stages).
Thus, there is a need for thermally conductive filler materials which can be used at low loading levels to achieve sufficient thermal conductivity. The present invention is directed to overcoming this deficiency in the art.
The present invention relates to a powder including agglomerates of hexagonal boron nitride platelets, wherein the agglomerates have an agglomerate size distribution of from about 10 to about 125 microns and the powder is substantially free of non-agglomerated platelets of boron nitride.
The present invention also relates to a method of making a powder including agglomerates of hexagonal boron nitride platelets. This method involves providing a briquette formed from agglomerated hexagonal boron nitride platelets, crushing the briquette to produce a powder including hexagonal boron nitride agglomerates and non-agglomerated boron nitride platelets, and removing the non-agglomerated boron nitride platelets from the powder under conditions effective to produce a powder including agglomerates of hexagonal boron nitride platelets, wherein the agglomerates have an agglomerate size distribution of from about 10 to about 125 microns.
Another aspect of the present invention relates to a polymer blend including a polymer and a powder phase. The powder phase includes agglomerates of hexagonal boron nitride platelets, wherein the agglomerates have an agglomerate size distribution of from about 10 to about 125 microns and the powder phase is substantially free of non-agglomerated platelets of boron nitride and wherein the powder phase is distributed homogeneously within the polymer.
The boron nitride powder of the present invention exhibits high thermal conductivity when used as a filler for thermal management applications, e.g., in composites, polymers, and fluids. In addition, the high thermal conductivity of the boron nitride powder of the present invention allows lower loadings, resulting in improved peel strengths of metal films bonded to polymer matrices. Moreover, the boron nitride powder of the present invention reduces viscosity in a boron nitride/polymer mixture, which leads to improved processing and wetting of the boron nitride by the polymer. The resulting boron nitride powder can also be used in composite polymers and metals for friction applications.